1. Field of the Invention
The present invention generally relates to magnet controls and, more particularly to a solid-state magnet control.
2. Description of Related Art
Electromagnets for use in lifting, crane, and/or any other similar operations are well known and are used for the positioning and handling of ferromagnetic materials including steel products or the like in a variety of arrangements including processing machines, pick-and-place units, automation projects, etc. Such electromagnets can be used in the lifting of flat, round, and/or other shapes of ferromagnetic materials. Operating modes for use with lifting magnets typically include LIFT, DROP, and/or DRIBBLE operating modes.
Magnet controllers using existing technology typically require the input power supply voltage to be above a minimum level in order to maintain holding of the load and to provide control power voltage to their contactor coils. Upon loss of input power, or a reduction below that required to hold in the contact coils of such magnet controllers, then the circuit of such magnet controllers is configured, by their default de-energized condition, into the DROP configuration. The magnet energy is immediately discharged through a discharge load and the load is dropped.
Conventional magnet controllers discharge the energy stored within the magnet by switching a discharge load across the magnet. Common discharge loads utilized are resistors or varistors. The problem with conventional magnet controllers is that the rapid change in magnet current (di/dt), caused when the load is switched across the magnet, induces a very high magnitude voltage spike into the supply. These discharge voltage spikes can range from around 800 VDC to greater than around 1,500 VDC depending on the discharge load utilized and these voltage spikes can cause damage and deterioration of equipment.
Using traditional voltage control contactor based magnet controllers, magnets heat up during use due to internal electrical power losses proportional to the resistance of the magnet coil. The rise in magnet temperature causes an increased resistance of the copper or aluminum magnet windings and, subsequently, causes proportionally greater internal power losses. The increased resistance reduces the current acceptance of the magnet coil as well as the magnitude of the current through the magnet as defined by V=IR when utilizing, a constant voltage supply. The reduced current acceptance reduces the capacity of the magnet, the magnet is able to pick up less material, and the reduced efficiency results in reduced production throughput.
Magnet charge up time is the time it takes for the magnet current to build up to the steady state lifting current level. Magnet charge up time, for a given magnet, is dependent upon the voltage applied. The higher the supply voltage, the less time it takes to build up lifting current within the magnet. Traditional technology uses a constant voltage supply, typically 230 VDC, to supply power to the magnet. Because traditional technology does not control magnet current, use of a higher voltage supply would induce magnet currents greater than the magnet can withstand.
Therefore, a need exists for a solid-state magnet control to reduce maintenance requirements, improve production efficiency, and provide current control, as well as inhibit circuit damage to magnet control circuitry and other component contained within a magnet control caused by overvoltage and/or voltage transients.